1. Field of the Invention
This invention relates generally to a pulse code modulation (PCM) data enhancer for improving transmission accuracy. More particularly, this invention relates to detecting transmission errors occurring when PCM data words are part of an underlying waveform for which correlations can be determined.
2. Discussion
Pulse code modulation (PCM) is a method of transmitting continuously varying signals in which a signal is sampled, the value of each sample is quantized into one of a finite set of permitted values, and the quantized value is then transmitted in patterns of on-or-off digital pulses or bits. These operations are traditionally known as sampling, quantizing, and coding. A typical PCM system transmits a large number of on-or-off pulses per second. In a telephone conversation, for example, 56,000 on-or-off pulses per second may be transmitted to effect PCM communication. In a one-way television communication operation, 70,000,000 on-or-off pulses per second may transmit the television signal. High transmission speeds enable extremely fast communication. Furthermore, a relatively high quality signal is possible even though noise and interference make it difficult to recognize the incoming pulses because only digital logic levels must be detected. However, even high quality signals experience some loss of data while operating in noise and interference conditions. Data loss may also occur when operating in relatively low signal-to-noise ratio situations, as well. When this occurs, recovering lost data is essential to a quality communication system.
Pulse code modulation data has traditionally been relatively sensitive to data loss when bit errors do occur during transmission. In a typical PCM system, even a moderate bit error rate can cause unacceptable noise in the received data signal. In some PCM systems, error correcting codes can detect bit errors, but these systems generally require cooperation between the transmitting and receiver data devices. In other systems, noise reduction occurs by improving the quality of the transmitting and receiving equipment, but such modifications are relatively costly to effect. Moreover, when the sending and receiving equipment cooperate to provide improved PCM signal quality transmission, additional overhead and communication between sending and receiving equipment is required, which further increases the cost of the equipment and decreases the speed of the PCM communication system.
Thus, it is desirable to provide a PCM communication system in which data transmission bit errors are correctable using existing equipment, without significantly increasing the processing or communication overhead required by the sending and receiving communication devices.